Solid state imaging devices contain an array of many thousands of individual imaging elements along with means for sampling the imaging elements in succession to produce a raster scan. Whether the imaging elements are sensitive to optical, infra-red, ultra-violet, X-ray, gamma-ray or other electromagnetic energies, or to sonic or charged particle energies, a common problem in the fabrication of these imaging devices is due to microscopic defects that cause a small percentage of the imaging elements or the scanning circuitry to be defective. Thus, an array comprising many thousands of elements must be discarded because of the defects in only a few of the elements. At the present time, as many as 95% of the complete imaging devices produced must be discarded as partially defective.
Several methods have been employed in the past to try to allow the use of partially defective imaging devices. One method, taught for example by F. Nagumo et al in U.S. Pat. No. 4,167,754, provides a memory for storing the addresses of imaging defects, and means to substitute some combination of neighboring image values for the defective points. Such methods can provide only an approximation to the missing image point.
Another method makes use of a beam splitter to split the incident energy into two beams incident upon two partially defective imaging devices. As taught, for example in U.S. Pat. No. 4,488,178 to J. Koslov et al for the optical case, such a method can be effective in reducing the imaging defects if the two imaging devices contain non-coincident defects. One major problem with such methods concerns registration of the two devices. Another problem is that two imaging devices must be employed for each imaging system, thereby restricting this method of increasing the yield of imaging devices to at most 50%.
In a third method, more akin to the present invention, a dither motion is provided in the image relative to the imaging device. As disclosed in a recent U.S. Pat. No. 4,581,649 to S. Morokawa by vibrating the sensor or the image at a single frequency or a combination of pseudo-random frequencies, and taking samples of the image points at different phases in the motion of the image, a series of values from different imaging elements can be obtained for the same image point. These values are compared to produce (by majority decision, e.g.) a best value for the image point.
Although the patent to Morokawa discloses a first step in the productive utilization of the concept of the shifting the image across the imaging device for the purpose of providing the redundancy necessary for reducing the effect of imaging defects, many problems still exist in the system presented by Morokawa.
Most importantly, because the scans A, B, C, and D occur after periods of motion of the imager or image (represented e.g. by the time between successive sampling clock pulses CLA and CLB of FIG. 6B), the resultant image signal will be an average over the motion during that time. In other words, the resultant signal will be blurred. It should be noted that with current technology and at standard scan rates, the image sensors require a substantial portion of the time between each sampling in order to obtain a sufficient number of optical photons (or X-rays, phonons, etc.) to produce a viable signal. If the displacement of the image relative to the imager is as large as shown in FIG. 1 during that time, the blurring will be across a large portion of the image. Moreover, because the separate scans are taken at different phases of the motion of the imager, the blurring will be in different directions. As shown in FIG. 6B, blurring will be in one direction for scans B and C, and the opposite direction for scans D and A. Thus, the majority logic or other combination means for determining the correct image data will be frustrated, since the signals B, C and A, D will be substantially different.
Other prior art considered to be relevant to the present invention are the following U.S. patents:
U.S. Pat. No. 4,595,954 Y. Endo et al, 358/213 PA1 U.S. Pat. No. 4,567,524 P. Levine, 358/213 PA1 U.S. Pat. No. 4,543,601 N. Harada et al, 358/213 PA1 U.S. Pat. No. 4,541,016 S. Ochi et al, 358/213 PA1 U.S. Pat. No. 4,535,363 N. Harada et al, 358/213 PA1 U.S. Pat. No. 4,383,170 N. Takagi et al, 250/216 PA1 U.S. Pat. No. 4,288,817 A. Igel, 358/163 PA1 U.S. Pat. No. 2,026,376 S. Colgate
The patents to Endo and Harada are of interest in that they disclose imaging systems in which the motion of the image relative to the imaging elements follows a trajectory of sudden motion and a subsequent long stationary phase. In addition, these patents further disclose shutter means for blocking the image from reaching the imaging elements during the period of motion. However, these patents are concerned only with increasing the resolution of a perfectly functional imaging device rather than the use of a device having defective portions. In addition, no means for compensating for the shift in location of the image relative to the imaging elements is disclosed.